Two of the characteristics of life are the establishment and maintenance of an intracellular ionic composition markedly different from the cell external environment, and the existence of multiple exchanges of ions and molecules across the plasma membrane. The regulation of intracellular ionic composition is a main determinant of cellular salt and water homeostasis, of cellular volume, and of vectorial net transport in epithelia. Intracellular ionic composition may play an important role in: initiation of DNA synthesis, cellular division, differentiation, transformation, polarity and cell to cell communication. Knowledge of composition and rates of change of intracellular ionic contents under hormones, drugs, physiological and pathological conditions is key to the understanding of the biology of a cell. The National Electron Probe Resource for Analysis of Cells will allow researchers to measure ionic contents and their rates of change in any type of cell. The cells will be cultured on small segments of polygraphite or silicon, and will remain on the same support for experimentation, preparation and electron probe analysis. Complete ionic balance will be provided: K, Na, Cl, Mg, Ca, P, S, and C will be measured simultaneously on individual cells. A very small number of cells will be sufficient for complete kinetic analysis; for sorting out different functional responses, for comparing cytoplasmic to nuclear content, for studying the effect of contact between cells. Cells originating from a selected part microdissected from an organ will be accessible to analysis in primary cultures. Microscopy time-lapse video recording will relate the analysis of the ionic content to observations on living cells. Electron probe autoradiograph will provide a powerful method to correlate ionic content and phase of the cell cycle determined on individual cells and may be applied to the study of metabolites transport in and among cells. Analysis of frozen hydrated cells and development of special supports will allow researchers to study particular aspects of cell and tissue polarity. Microfluorescence methods will allow intracellular pH changes to be followed on individual cells. This Resource shall have a major impact on the study of ionic cell permeability and transport; their regulation; their role in cellular salt and water homeostasis, on cell volume maintenance and cell volume regulation; their variations during the cell cycle, cell growth, cell division, differentiation, and transformation; their possible perturbation in genetic disorders like essential hypertension, cystic fibrosis, Duchenne muscular dystrophy; and their role in injury following cellular anoxia.